1. Field of the Invention
The present invention relates to an Active Matrix Organic Light Emitting Diode (AMOLED) display device, and more particularly, to an AMOLED display device which can sense a current of each pixel at high speed by a simple structure in order to compensate for a luminance deviation between pixels and a pixel current sensing method thereof.
2. Discussion of the Related Art
An AMOLED display device is a self-luminous element in which an organic light emitting layer emits light by recombination of electrons and holes and is expected to be used as a future generation display device due to high luminescence, a low driving voltage, and ultra thin thickness.
Each of a plurality of pixels constituting the AMOLED display device includes an Organic Light Emitting Diode (OLED) comprised of an organic light emitting layer between an anode and a cathode and a pixel circuit for independently driving the OLED. The pixel circuit mainly includes a switching Thin Film Transistor (TFT), a capacitor, and a driving TFT. The switching TFT charges a voltage corresponding to a data signal to a capacitor in response to a scan pulse and the driving TFT adjusts the amount of light emission of the OLED by controlling the magnitude of current supplied to the OLED according to the magnitude of the voltage charged in the capacitor. The amount of light emission of the OLED is proportional to current supplied from the driving TFT.
However, in the OLED display device, there is a characteristic difference in a threshold voltage Vth and mobility of the driving TFT between pixels due to a process deviation etc. and thus the amount of current for driving the OLED varies. As a result, a luminance deviation occurs between pixels. Generally, an initially generated characteristic difference of a driving TFT results in spots or patterns on a screen and a characteristic difference caused by degradation of the driving TFT produced while the OLED is driven reduces the lifespan of an AMOLED display panel or generates a residual image.
To solve such a problem, for example, U.S. Pat. No. 7,838,825 discloses a data compensation method for compensating for input data according to a result obtained by sensing a current of each pixel. However, the above patent in which a method for sensing a current flowing to a power line (VDD or VSS) of a panel while lighting up each pixel is used delays a current sensing time due to a parasitic capacitor existing in parallel to the power line when resolution is increased, thereby making it difficult to perform high-speed current sensing.
In addition, although currents of a plurality of pixels can simultaneously sense at high speed using a plurality of current sensing circuits, circuit size is increased and therefore such a method is not practical. Due to this, the above patent can compensate for an initially generated characteristic deviation between driving TFTs by sensing the deviation in a test process prior to product shipment. However, it is difficult to sense and compensate for a characteristic deviation due to degradation of the driving TFT generated while the OLED is driven after product shipment.